1. Field of Invention
The present invention relates to a frequency-converting circuit, and more particularly, to a frequency-converting circuit using a transformer-coupling approach.
2. Description of the Related Art
In recent years, a so-called direct-conversion receiver (termed as homodyne receiver or zero-IF receiver, too) has become more important by its high performances, such as architecture simplicity and monolithic design, in the field of transceivers. However, some drawbacks of the conventional direct-conversion receiver resulted from the adopted scheme of one-off zero-IF operation are to be overcome. For example, the frequency of the poly-phase local oscillating signal (poly-phase LO signal) generated by such sort of receiver is very close to the one of the radio frequency signal (RF signal). In addition, the isolation between both of the mixer in the receiver and the input terminal jointly referred to “RF signal input terminal” herein) of the low noise amplifier (LNA) and the poly-phase LO signal received by the mixer is not infinite. Therefore, a poly-phase LO signal present at the RF signal input terminal through a feedthrough effect would be self-mixed with the original poly-phase LO signal, which further causes a DC offset to affect the originally received RF signal.
For solving the above-mentioned problem, the conventional down converter employs a conventional sub-harmonic mixer (SHM) with high isolation characteristic to decrease the DC offset, as shown in FIG. 1. Wherein, conventional SHMs 102 and 103 are circuits with double input ports and double output ports so that the received differential input signals, i.e. the RF signals RFI1 and RFQ1, are the ones generated by a quadrature coupler 101 receiving the RF signal RF1. At the point, for the down converter to obtain two baseband signals BF11 and BF12 which are quadrature to each other, a phase shifter 104 must provide two sets of poly-phase LO signals PLO 11 and PLO12. The poly-phase LO signal PLO11 herein includes local oscillating signals (LO signals) LO1˜LO4, which have phase displacements of 0°, 90°, 180° and 270°, respectively. The poly-phase LO signal PLO12 includes LO signals LO5˜LO8, which have phase displacements of 45°, 135°, 225° and 315°, respectively. To generate eight LO signals LO1˜LO8 with different phase displacements, the conventional down converter is required to employ the phase shifter 104 with a remarkably complex structure. In other words, a conventional down converter with reduced DC offset by using conventional SHMs results in a dilemma where the advanced electric behavior of the circuit is associated with increased circuit complexity and chip area consumption.